In recent years, nonaqueous electrolyte secondary batteries have been widely used as power supplies for driving portable electronic devices such as smartphones, tablet computers, notebook personal computers, and portable music players. Furthermore, applications of the nonaqueous electrolyte secondary batteries extend to electric tools, power-assisted bicycles, and electric vehicles; hence, the nonaqueous electrolyte secondary batteries are required to have high capacity and high output.
A carbon material such as graphite is used as a negative electrode active material for the nonaqueous electrolyte secondary batteries. The carbon material has a discharge potential comparable to that of metallic lithium and can suppress the dendritic growth of lithium during charge. Therefore, using the carbon material as a negative electrode active material enables nonaqueous electrolyte secondary batteries excellent in safety to be provided. Graphite can store lithium ions to form the composition LiC6 and exhibits a theoretical capacity of 372 mAh/g.
However, carbon materials currently used already exhibit a capacity close to the theoretical capacity thereof; hence, it is difficult to increase the capacity of nonaqueous electrolyte secondary batteries by improving negative electrode active materials. Therefore, in recent years, silicon materials, such as silicon and oxides thereof, having a capacity higher than that of the carbon materials have been attracting attention as negative electrode active materials for nonaqueous electrolyte secondary batteries. For example, silicon can store lithium ions to form the composition Li4.4Si and exhibits a theoretical capacity of 4,200 mAh/g. Therefore, using the silicon materials as negative electrode active materials allows nonaqueous electrolyte secondary batteries to have increased capacity.
The silicon materials, as well as the carbon materials, can suppress the dendritic growth of lithium during charge. However, the silicon materials show a larger change in volume due to charge and discharge as compared to the carbon materials and therefore have a problem of inferior cycle characteristics because of the pulverization of negative electrode active materials, the peel-off from conductive networks, or the like.
Patent Literature 1 discloses a nonaqueous electrolyte secondary battery including a negative electrode mixture layer containing a material containing Si and O as constituent elements and graphite as negative electrode active materials and a positive electrode mixture layer containing a lithium-transition metal composite oxide in which Ni, Mn, and the like are essential constituent elements as a positive electrode active material. It is reported that a nonaqueous electrolyte secondary battery having high capacity and good battery characteristics is obtained by regulating the proportion of the material containing Si and O as constituent elements in a predetermined range.
As a technique for enhancing output characteristics of a nonaqueous electrolyte secondary battery, Patent Literature 2 discloses that negative electrode tabs are connected to regions, set at both ends of a negative electrode plate of the nonaqueous electrolyte secondary battery, uncoated with a negative electrode active material.